1. Field of the Invention
This invention relates to the field of using magnetic sensors to measure torque applied to a rotatable shaft. More particularly, the invention relates to the particular placement of magnetic field sensors in relation to a magnetic transducer element to measure the torque applied to a shaft, and the sensor system is immune to external magnetic sources within a large temperature range.
2. Prior Art
Magnetic torque sensors are known in the art. Many versions of this type of magnetoelastic torque sensor have been proposed and described in a plurality of publications and patent specifications. There are two general ways of utilizing the magnetoelastic phenomenon as the sensing mechanism for a non-contacting torque sensor. One is permeability based, and the other is polarized band based.
In permeability based types of magnetoelastic torque sensors, the permeability changes in the shaft surface, caused by the stress-induced magnetic anisotropy, affect the permeance of a magnetic flux path which includes a magnetizing source and a sensing coil. In these type of devices, the magnetic property effectively sensed is a permeability μ, of one form or another, the output signals are derived from a magnetic flux density B of a flux that arises in response to an excitation field H with B=μH, where μ is altered by the stress and hence by the transmitted torque. This results in different voltages being induced in the sense winding encircling the two regions and this difference provides the measure of the torque. This kind of transducer comprises a shaft with surrounding excitation windings and measuring windings. Concentrically with the windings, anisotropic measuring zones are created in the shaft by different methods, e.g., by attaching a sleeve of magnetic material, formed with two parallel, annular zones which are provided with slits having a substantially regular pitch and making an angle of ±45° with a generatrix of the envelope surface of the sleeve (U.S. Pat. No. 4,506,554 to Blomkvist et al.) or by applying layers, such as copper strips, on the surface of the shaft (U.S. Pat. No. 5,646,356 to Ling et al.). Another method is forming a series of parallel grooves and/or lands or protuberances inclined at a suitable angle with the longitudinal axis of the shaft (U.S. Pat. No. 4,823,620 to Edo et al.) or a shaft having a magnetostrictive film (Pub. No. U.S. 2007/0245833 A1 to Yoneda et al.), or a pair of amorphous magnetic ribbons bonded to a shaft (U.S. Pat. No. 4,907,462 to Obama et al.).
An example of a commercially offered permeability based magnetoelastic torque transducer is that offered under the Trade Mark Torductor by the Force Measurement division of ABB AB, S-721 59 Vasteras, Sweden.
In polarized band types of magnetoelastic torque sensors, sensor operation is based on the reorienting effects of torsional stress on the individual magnetic moments that have been remanently circularly magnetized. In response to the magnetoelastic energy associated with the biaxial principal stresses by which torque is transmitted along the shaft, each moment will rotate towards the nearest positive principal stress direction and away from the nearest negative principal direction. This reorientation of the originally circular magnetization results in a net axial magnetization component. The divergence of this component at the edges of the polarized bands is the source of a magnetic field in the space around the shaft that can be readily measured with one or more magnetic field sensors. This type of transducer can be constructed either with a thin ring of magnetoelastically active material rigidly attached to the shaft (U.S. Pat. No. 5,351,555 to Garshelis), or by using a portion of the solid or hollow shaft itself as the magnetoelastically active element (U.S. Pat. No. 6,047,605 to Garshelis, and U.S. Pat. No. 6,581,480 B1 to May et al.). The magnetoelastic zone may also be remanently longitudinally magnetized.
An example of a commercially offered polarized band based magnetoelastic torque transducer is that offered by the Magna-Lastic Devices, Inc, a division of Methode Electronics, Inc, 7401 West Wilson Avenue, Chicago, Ill.
The basic weakness in these prior art approaches to magnetoelastic torque transducers is that they can not distinguish whether the sensed quantity, i.e., magnetic flux B, is depended on torsional stress induced by applied torque or an external magnetic flux source, or a temperature dependent magnetic flux, thus with the undesirable result that the sensed variations in B do not unambiguously indicate a variation in torque.
Most prior art devices employ a construction of common mode rejection to filter out uniform external magnetic flux, known as far field, e.g. magnetic field, by providing two distinct B dependent signals, having equal quiescent values, but opposite responses to torque, with means for combining the two signals differentially. This demands great care in matching the sensitivity of the two magnetic flux B sensors. This configuration can not eliminate non-uniform external magnetic flux sources, known as near field, such as a nearby electric motor, or a nearby moving ferromagnetic object.
Another attempt to avoid external magnetic flux is to position a magnetic shield around the transducer to protect it from external magnetic sources, as detailed in U.S. Pat. No. 5,083,359 to Aminder et al. and U.S. Pat. No. 5,889,215 to Kilmartin et al.
Yet another attempt to compensate the interference field was by energizing coils to provide a counteracting magnetic field, as detailed in U.S. Pat. No. 6,826,969 B1 to May.
Unfortunately, providing sufficient space for the requisite shield and counteracting coils has created practical problems in applications where space is at a premium, also, such a shield appears to be impractically expensive for using on highly cost-competitive devices, such as in automotive applications, and these approaches increase the complexity and the cost of a complete sensor while also generally reducing its adaptability, maintainability and reliability. These approaches also do not compensate for the temperature offset effect and rotation-dependent output.